The search for new therapeutic agents has been greatly aided in recent years by a better understanding of the structure of enzymes and other biomolecules associated with target diseases. One important class of enzymes that has been the subject of extensive study is protein kinases.
Protein kinases mediate intracellular signal transduction. They do this by effecting a phosphoryl transfer from a nucleoside triphosphate to a protein acceptor that is involved in a signaling pathway. There are a number of kinases and pathways through which extracellular and other stimuli cause a variety of cellular responses to occur inside the cell. Examples of such stimuli include environmental and chemical stress signals (e.g., osmotic shock, heat shock, ultraviolet radiation, bacterial endotoxin, and H2O2), cytokines (e.g., interleukin-1 (IL-1) and tumor necrosis factor α (TNF-α)), and growth factors (e.g., granulocyte macrophage-colony-stimulating factor (GM-CSF), and fibroblast growth factor (FGF)). An extracellular stimulus may affect one or more cellular responses related to cell growth, migration, differentiation, secretion of hormones, activation of transcription factors, muscle contraction, glucose metabolism, control of protein synthesis and regulation of cell cycle.
Many disease states are associated with abnormal cellular responses triggered by protein kinase-mediated events. These diseases include autoimmune diseases, inflammatory diseases, metabolic diseases, neurological and neurodegenerative diseases, cancer, cardiovascular diseases, allergies and asthma, Alzheimer's disease and hormone-related diseases. Accordingly, there has been a substantial effort in medicinal chemistry to find protein kinase inhibitors that are effective as therapeutic agents. A challenge has been to find protein kinase inhibitors that act in a selective manner. Since there are numerous protein kinases that are involved in a variety of cellular responses, non-selective inhibitors may lead to unwanted side effects.
Glycogen synthase kinase-3 (GSK-3) is a serine/threonine protein kinase comprised of α and β isoforms that are each encoded by distinct genes [Coghlan et al., Chemistry & Biology, 7, 793-803 (2000); and Kim and Kimmel, Curr. Opinion Genetics Dev., 10, 508-514 (2000)]. GSK-3 has been implicated in various diseases including diabetes, Alzheimer's disease, CNS disorders such as manic depressive disorder and neurodegenerative diseases, and cardiomyocete hypertrophy [see, e.g., WO 99/65897; WO 00/38675; and Haq et al., J. Cell Biol. (2000) 151, 117]. These diseases may be caused by, or may result in, the abnormal operation of certain cell signaling pathways in which GSK-3 plays a role.
GSK-3 has been found to phosphorylate and modulate the activity of a number of regulatory proteins. These include glycogen synthase, which is the rate-limiting enzyme required for glycogen synthesis, the microtubule-associated protein Tau, the gene transcription factor β-catenin, the translation initiation factor e1F-2B, as well as ATP citrate lyase, axin, heat shock factor-1, c-Jun, c-myc, c-myb, CREB, and CEPBα. These diverse targets implicate GSK-3 in many aspects of cellular metabolism, proliferation, differentiation and development.
In a GSK-3 mediated pathway that is relevant for the treatment of type II diabetes, insulin-induced signaling leads to cellular glucose uptake and glycogen synthesis. GSK-3 is a negative regulator of the insulin-induced signal in this pathway. Normally, the presence of insulin causes inhibition of GSK-3-mediated phosphorylation and deactivation of glycogen synthase. The inhibition of GSK-3 leads to increased glycogen synthesis and glucose uptake [Klein et al., PNAS, 93, 8455-9 (1996); Cross et al., Biochem. J., 303, 21-26 (1994); Cohen, Biochem. Soc. Trans., 21, 555-567 (1993); and Massillon et al., Biochem J. 299, 123-128 (1994)]. However, where the insulin response is impaired in a diabetic patient, glycogen synthesis and glucose uptake fail to increase despite the presence of relatively high blood levels of insulin. This leads to abnormally high blood levels of glucose with acute and chronic effects that may ultimately result in cardiovascular disease, renal failure and blindness. In such patients, the normal insulin-induced inhibition of GSK-3 fails to occur. It has also been reported that in patients with type II diabetes, GSK-3 is overexpressed [WO 00/38675]. Therapeutic inhibitors of GSK-3 are therefore potentially useful for treating diabetic patients suffering from an impaired response to insulin.
GSK-3 activity has also been associated with Alzheimer's disease. This disease is characterized by the presence of the well-known β-amyloid peptide and the formation of intracellular neurofibrillary tangles. The neurofibrillary tangles contain hyperphosphorylated Tau protein, in which Tau is phosphorylated on abnormal sites. GSK-3 has been shown to phosphorylate these abnormal sites in cell and animal models. Furthermore, inhibition of GSK-3 has been shown to prevent hyperphosphorylation of Tau in cells [Lovestone et al., Current Biology 4, 1077-86 (1994); and Brownlees et al., Neuroreport 8, 3251-55 (1997)]. Therefore, it is believed that GSK-3 activity may promote generation of neurofibrillary tangles and progression of Alzheimer's disease.
Another substrate of GSK-3 is β-catenin, which is degradated after phosphorylation by GSK-3. Reduced levels of β-catenin have been reported in schizophrenic patients and have also been associated with other diseases related to increase in neuronal cell death [Zhong et al., Nature, 395, 698-702 (1998); Takashima et al., PNAS, 90, 7789-93 (1993); Pei et al., J. Neuropathol. Exp, 56, 70-78 (1997); and Smith et al., Bio-org. Med. Chem. 11, 635-639 (2001)].
Small molecule inhibitors of GSK-3 have recently been reported [WO 99/65897 (Chiron) and WO 00/38675 (SmithKline Beecham)]; however, there is a continued need to find new therapeutic agents to treat human diseases. The protein kinase GSK-3, in particular GSK-3β, is an especially attractive target for the discovery of new therapeutics due to its important role in diabetes, Alzheimer's disease and other diseases.
Aurora-2 is another serine/threonine protein kinase that has been implicated in human cancer, such as colon, breast and other solid tumors. This kinase is believed to be involved in protein phosphorylation events that regulate the cell cycle. Specifically, Aurora-2 may play a role in controlling the accurate segregation of chromosomes during mitosis. Misregulation of the cell cycle can lead to cellular proliferation and other abnormalities. In human colon cancer tissue, the Aurora-2 protein has been found to be overexpressed [Bischoff et al., EMBO J., 17, 3052-3065 (1998); Schumacher et al., J. Cell Biol., 143, 1635-1646 (1998); and Kimura et al., J. Biol. Chem., 272, 13766-13771 (1997)].
Cyclin-dependent kinases (CDKs) inhibitors have been described as anticancer agents [Fischer, P. M. and Lane, D. P., Current Medicinal Chemistry, 7, 1213-1245 (2000); Mani, S., Wang, C., Wu, K., Francis, R. and Pestell, R., Exp. Opin. Invest. Drugs, 9, 1849 (2000); and Fry, D. W. and Garrett, M. D., Current Opinion in Oncologic, Endocrine & Metabolic Investigational Drugs, 2, 40-59 (2000)]. Although some CDK-2 inhibitors have been disclosed, it would be desirable to have other CDK-2 inhibitors for treating human diseases.
Therefore, there is a need to inhibit protein kinases, particularly glycogen synthase kinases, Aurora and cyclin-dependent kinases, more particularly GSK-3, Aurora-2 and CDK-2, for treatment of human diseases.